1. Field of the Invention
The present invention relates to lasers and more particularly to terahertz imaging systems.
2. Brief Description Of Prior Developments
Research into optical terahertz generation has gained interest due to its imaging and spectroscopic applications, as is disclosed in Y. Chen et al., “THz diffuse reflectance spectra of selected explosives and related compounds,” in Terahertz for Military and Security Applications , edited by R. Jennifer Hwu, Dwight L. Woolard, Mark J. Rosker, Proceedings of the SPIE Vol. 5790 (SPIE, Bellingham, Wash.), May 2005; Jason C. Dickinson et al., “Terahertz imaging of subjects with concealed weapons,” in Terahertz for Military and Security Applications IV , edited by Dwight L. Woolard, R. Jennifer Hwu, Mark J. Rosker, James O. Jensen, Proceedings of the SPIE Vol. 6212 (SPIE, Bellingham, Wash., 2006), May 2006; and A. W. M. Lee and Q. Hu, “Real-time, continuous-wave terahertz imaging by use of a microbolometer focal-plane array,” Opt. Lett., vol. 30, no. 19, pp. 2563-2565, October 2005, the contents of all which are incorporated herein by reference. Specifically, the ability to pass through clothing and various other materials without high levels of attenuation allows for imaging of concealed weapons. This ability is particularly of interest for real-time security screening. Concealed weapons detection is made possible by the transparency of clothing and fibrous materials in the THz region along with the high reflectance of metallic and dense objects. In addition, THz radiation is non-ionizing, which allows for safe illumination of human subjects. Potential applications include mail screening, luggage screening at airports, and concealed weapons screening on people. This technology may eventually reduce security bottlenecks and may also allow for the detection of non-metallic weapons made from fiberglass or other dense plastics that are invisible to metal detectors. Terahertz may also be used to detect various explosive and caustic substances, as is disclosed in H. Liu, Y. Chen, G. J. Bastiaans, and X.-C. Zhang, “Detection and identification of explosive RDX by THz diffuse reflection spectroscopy,” Opt. Express, vol. 14, no. 1, pp. 415-423, January 2006, the contents of which are incorporated herein by reference, providing complete security screening capability in one packaged system.
Pulsed Yb-doped fiber systems have been successful in generating diffraction-limited, high peak-power, 1064 nm pulses at various pulse widths and repetition rates, as is disclosed in D. Creeden et al., “Near diffraction-limited, 1064 nm, all-fiber master oscillator fiber amplifier (MOFA) with enhanced SRS suppression for pulsed nanosecond applications,” presented at the 2006 SSDLTR, Albuquerque, N. Mex., Jun. 13-15, 2006, Paper FIBER1-4; and D. Creeden et al., “Fiber laser transmitter for LADAR applications,” in Proc. 2006 Meting of the MSS Specialty Group on Active E-O Systems, Monterey, Calif., 2006, the contents of which are incorporated herein by reference. This flexibility makes pulsed fiber sources ideal for driving frequency conversions in nonlinear crystals. Additionally, Yb-doped fiber has a large gain-bandwidth that allows for amplification over a large spectral range. This enables the amplification of two wavelengths simultaneously in a single gain fiber. Because of this, a single fiber laser system can be used to amplify two different signals which can then be difference frequency mixed in a nonlinear crystal to generate terahertz radiation. This reduces the complexity and cost of typical DFM based terahertz sources.
The development of nonlinear materials has also enabled efficient generation of THz radiation, as is disclosed in P. A. Budni, L. A. Pomeranz, M. L. Lemons, P. G. Schunemann, T. M. Pollak, and E. P. Chicklis, “10 W mid-IR holmium pumped ZnGeP2 OPO,” in OSA Trends in Optics and Photonics, vol. 19, pp. 226-229 (1998), the contents of which are incorporated herein by reference. Zinc Germanium Phosphide (ZGP) combines high nonlinear gain and birefringence with relatively low THz loss. In addition, ZGP can be engineered to have low absorption in the 1 micron region, which makes it an ideal crystal for THz generation.
A need still exists, however, for an improved way to practice real-time terahertz imaging for the detection of concealed weapons or other concealed objects.